Communication cables that include optical fibers have been deployed in many types of installations. For example, fiber optic cables are often installed underground, either by burying them directly or by blowing them through ducts. Another installation option has been to string the cables aerially between poles, as with traditional telephone lines.
Of these methods, aerial installation has gained popularity. It costs less to deploy cables above ground than below ground, and aerial installation makes the fiber optic cable easier to access for maintenance or repair. Moreover, cables installed above ground tend to be less susceptible to damage, which may happen to cables installed in ground by unintentional excavation.
While optical fiber cables are typically installed aerially by suspending them between poles, this technique applies stresses to the cable that cables in other installations do not face. For instance, aerial installation imparts substantial tensile stresses on the cable caused by the weight of the cable suspended between poles. Wind, snow, and ice can increase these stresses. Exposure to the environment also can subject the cable to thermo stresses from the climate. The tensile and thermo stresses can increase attenuation in the optical fibers, adversely impacting their performance as a communication medium. Lashing the cable to suspension wires may decrease tensile stresses, but it introduces other problems. Namely, suspension wires significantly increase the cost of installation and, as conductors, may attract lightning. Lightning strikes can seriously damage the fiber optic cable.
In short, fiber optic cables installed aerially need to withstand the increased stresses that arise from suspension and need to avoid attracting lightning strikes. Conventional cables of this type are typically of the loose-tube design, where the fibers are housed in a plurality of buffer tubes stranded around a central strength member. The loose-tube design permits the fibers to move within the buffer tubes and avoid absorbing stress or strain on the cable. Moreover, the materials in the cable are exclusively dielectric to avoid lightning and allowing the cable to be placed in the power region of the pole. The cables are, therefore, called all-dielectric, self-supporting (ADSS) cables.
ADSS cables are designed to reduce stresses on the optical fibers. Fiber strain is a loss mechanism in optical fibers that may occur if the cable is subjected to tensile forces, either from installation or temperature, or compression forces. Fiber strain may cause signal loss in the optical fibers. A central strength member and usually outer strength members are included in ADSS cables to help bear the tensile and thermo stresses. Also, the optical fibers often have excess length so that they may move freely within the buffer tubes.
FIG. 1 shows a generalized cross-sectional view of a typical ADSS cable 102. ADSS cable 102 includes, at its core, a central strength member 104, which is capable of withstanding and controlling the significant tensile and thermo stresses that the ADSS cable may be subject to. Typically, central strength member 104 may be made from glass-fiber reinforced plastic. Central strength member 104 may have a jacket or coating 106 of polymeric material, such as, for example, a polyolefin or polyethylene coating.
A plurality of buffer tubes 108 surrounds central strength member 104. Each buffer tube 108 includes a plurality of optical fibers 110 within it. A gel-based filling material may be introduced inside buffer tube 108 to serve as a physical barrier to any water accidentally penetrated inside buffer tube 108. A water-swellable tape 112, an inner jacket 114 which is used to isolate the optical core, and outer strength members 116 respectively surround buffer tubes 108. An outer jacket 118 protects the exterior of the cable. A rip cord 120 provides a means for easily opening the cable jacket to access the fibers during installation or repair.
Known ADSS cables having the structure of FIG. 1 have had a maximum capacity of 288 fibers. Conventional ADSS cables with higher fiber counts have followed one of two alternative approaches.
In one design, shown in FIG. 2, a second layer of buffer tubes is added around the first layer. In this two-layered design, a first or inner layer of buffer tubes 202 is directly in contact with or stranded to central strength member 204, similar to the design in FIG. 1. To increase the fiber count, a second or outer layer of buffer tubes 206 is placed over and secured to the first or inner layer of buffer tubes 202. The buffer tubes in the second layer have substantially the same dimensions as the tubes in the first layer. A water blocking or swellable tape 208 may be inserted between the two layers 202 and 206. Other features of the two-layered design may be similar to those of the ADSS cable of FIG. 1.
In another design, loose fibers in the conventional ADSS cable of FIG. 1 are replaced with ribbon fibers. Optical fiber ribbons are planar arrays of fibers that are bonded together as a unit. Through bonding, ribbons provide a higher density of fibers per unit area. Ribbons can advantageously be mass fusion spliced, saving setup and maintenance costs. Consequently, for the same cable structure, an ADSS cable can generally provide a higher number of fibers using ribbons rather than loose or bundled fibers.
U.S. Pat. No. 6,185,351 describes an ADSS cable using ribbon fibers. FIG. 3 reproduces a cross-sectional view of the cable from the '351 patent. As shown in FIG. 3, stacks of ribbon fibers 302 are encased in six buffer tubes 304 in cable 300, leading to a total fiber count in excess of 288. Depending on the fiber count, the ribbon stacks 302 in cable 300 may be rectangular or square in shape. The ribbon stacks 302 are generally twisted into a helix to help maintain the stack form. Generally, the optical fibers of the ribbon stacks 302 are held together using an ultraviolet-curable matrix bonding material or other suitable boding material.
Applicants have noted that the known attempts for an ADSS cable having a fiber count in excess of 288 have several disadvantages. The two-layer design of FIG. 2, for example, exposes the optical fibers to excessive stress in an aerial installation. Specifically, being in immediate contact with central strength member 204, the first layer of buffer tubes 202 is generally well protected from tensile and thermo stresses from the environment. In static applications, such as in directly buried or duct applications, where straining of the cable is minimum, the second layer of buffer tubes 206 may also be adequately protected. However, the inner layer of buffer tubes can become decoupled from the outer layer and cause problems either immediately after installation or over time. Moreover, ADSS cables in aerial installations are subject to significant Aeolian vibration, direct exposure to hostile environmental conditions, and other conditions that create substantial tension and strain on the cable. In such strained conditions, there is less control over the expansion and/or contraction of the second layer of buffer tubes 206.
Additionally, securing a second layer of buffer tubes to an inner layer of buffer tubes causes extra stress and tension to be exerted on the inner layer of buffer tubes. Because an ADSS cable must carry the weight and installation tensions of the cable itself as well as the external loads created by the effects of wind and ice, the added stress from a second layer of buffer tubes is undesirable and may cause data attenuation and other unpredictable irregularities in the fibers in the inner layer of buffer tubes.
The design 300 using optical fiber ribbons also has several disadvantages. The fibers located at the corners of the stack may be subject to flexural stresses and may encounter friction from rubbing against the inner buffer tube walls. This may result in some unpredictable variations in attenuation in the corner fibers. One way to minimize this unpredictable attenuation of the corner fibers is to select corner fibers based on mode field diameter and cutoff wavelength. However, such selection is merely a way to minimize the impact of the problem associated with using ribbon stacks, not really solving the problem. Another disadvantage of using ribbon stacks is that the rigid shape of the ribbon arrangement minimizes excess fiber length that may be stored within the buffer tubes. Excess fiber length is desirable in ADSS cables. For example, fibers with excess length may move freely when exposed to environmental stresses and/or when exposed to manipulations such as when pulled out of a closure for the preparation of fiber ends for joining, or for other installation or maintenance related activities. Ribbon designs that have diminished excess fiber length are thus disadvantageous.
ADSS cables with ribbon fibers also suffer from having a comparatively small strain-free window. The strain-free window refers to the amount of axial load that can be applied to a cable before more than negligible amounts of strain (>0.1%) are imparted to the optical fibers within the cable.
Generally, cables with ribbon fibers in buffer tubes have smaller strain-free windows than cables with loose fibers in buffer tubes. The ribbon fibers are more constrained and cannot move as freely to avoid absorbing the strain placed on the cable.
The '351 patent in its FIG. 3 indicates that a high fiber count ADSS cable using ribbon fibers can achieve negligible strain on the optical fibers at about 0.18% cable strain. Moreover, it states that fiber strain increases optical attenuation and that the ADSS ribbon cable can achieve negligible attenuation for fiber strain up to approximately 0.275%. While the '351 patent discusses “packing density” and “clearance” in buffer tubes to permit fiber movement, Applicants have observed that achieving low-fiber strain in an ADSS cable having ribbon cables also requires large amounts of aramid fibers as an outer strength member system to attain a high enough modulus of elasticity for the cable to protect the fibers from stresses.
Applicants have noticed that the existing approaches for high fiber count ADSS cables do not provide a desirable balance between a large number of optical fibers in a single layer self-supporting cable and low susceptibility to strain on the optical fibers. Therefore, Applicants have perceived the need to provide a high fiber count ADSS cable which does not present the drawbacks of high fiber count ADSS cables known in the art wherein ribbon fibers or, alternatively, at least two layers of buffer tubes are used.